1. Field of the Invention
The present invention relates to a paper guide device for guiding a sheet of paper to an image transfer location. The invention particularly relates to paper guide devices adapted for use in an image forming apparatus such as a copier, printer, facsimile transceiver or similar image forming apparatus.
2. Description of the Related Art
FIG. 17 depicts a schematic sectional view of a copier. Referring to FIG. 17, a photoconductive drum 1 rotates in a direction indicated by the arrow. A main charger 2, an exposure device 3, a developer device 4, a transfer belt 5, a cleaning device 6 and a discharge lamp 7 are located around the photoconductive drum 1. A sheet of paper stored in a paper feed device 8 is fed toward register rollers 9. The sheet of paper which is at the register rollers 9 is then fed toward the photoconductive drum 1. The sheet of paper is guided by guide plates 10 and a thin straight plate 15 which is made of synthetic resin, and then goes into a transfer location. A toner image on the photoconductive drum 1 is transferred to the sheet of paper at the transfer location, and the sheet of paper is then transported by the transfer belt 5, and discharged to a tray 13 via a fixing device 11 and discharge rollers 13.
The paper feed device 8 is disposed under the photoconductive drum 1, so that the width dimension of the copier is relatively small. However, with the sheet of paper transported upward, the sheet of paper bends when it is fed from the register rollers 9 to the transfer location. If the sheet of paper bends, the rear end of the sheet of paper springs up when it passes through the thin plate 15. The spring action of the rear end of the sheet of paper can cause unusual or undesirable transfer of toner, and as a result, a toner image on the sheet of paper becomes poor. Springing of the rear end of the sheet of paper particularly occurs when the sheet of paper is thick.
FIGS. 18 and 19 depict a copier in which the sheet of paper is transported approximately perpendicular to the photoconductive drum 1. As shown in FIG. 19, if the sheet of paper is transported substantially perpendicular to the photoconductive drum 1, the rear end of the sheet of paper springs in a direction indicated by arrow P. As a result, a toner image on the sheet of paper becomes poor. Particularly, when thick paper (paper of 110 kg) is used, the rear end of the paper springs quite often.
Experimental runs were conducted with a copier as shown in FIG. 20 in which the sheet of paper S is transported almost perpendicular to the photoconductive drum 1 under normal temperature and humidity conditions. A Japanese postal card (paper of 110 kg) was used as the sheet of paper S. The result of the experiment is shown in FIG. 21. As shown in FIG. 21, a toner image at a location spaced from the rear end of the sheet of paper S by a distance q (10 mm to 15 mm) is poor. As shown in FIG. 20, the thin straight plate 15 is disposed on the guide plate 10. When the rear end of the sheet of paper S passes through the thin plate 15, the rear end of the sheet of paper S springs up, and then it strikes a surface of the photoconductive drum 1. When the rear end of the sheet of paper S passes through the thin plate 15, a sound is made, which is an indication that the rear end of the sheet of paper strikes the surface of the photoconductive drum 1. While thick paper passes along a nip portion between the photoconductive drum 1 and the transfer belt 5, the transfer belt 5 is pushed downward. As a result, the front end of the sheet of paper S separates from the transfer belt 5. Therefore, when the rear end of the sheet of paper S passes through the thin plate 15, it springs up hard.
In addition, toner on the surface of the photoconductive drum 1 is scattered by airflow which is generated by the rotation of the photoconductive drum 1. Scattered toner accumulates on the thin plate 15. Therefore, when the rear end of the sheet of paper S springs against the thin plate 15, toner on the thin plate 15 is scattered again, and then adheres to the sheet of paper S. As a result, the quality of a toner image becomes poor.